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Relative dating determines the order in which a sequence of past events occurred, but does not determine exactly when the geologic events happened.
Absolute age determinations are numeric and identify when, in years, specific events happened.
Both techniques are important in different geologic situations.
Yet, it is the canyon’s rock walls that allow people to develop their greatest perspective on geologic time, because of these rocks’ immense age, their fossil record, and because these rocks formed in environments far different than those found in northern Arizona today.
With a rock record that spans more than 1500 m.y., Grand Canyon is truly a panoramic view into the geologic past.
Someone looking at a photograph with men dressed in leisure suits would be able to correlate that photo to others of the same time period in which men are wearing leisure suits.
But to know that these photos were taken in the 1970’s requires another type of dating, an “absolute age determination.” Photo 4: Sedimentary rocks are typically deposited as vast horizontal layers.
For example, in Grand Canyon, there are many examples of sandstone deposition on a beach while limestone was forming off shore.
Geologists usually correlate sedimentary rocks based on their fossil assemblages, particularly on the presence of index fossils.
William “Strata” Smith first recognized in the early 1800s that the fossil record has changed systematically through time.
Smith used this observation to correlate rock units throughout England and make the world’s first geologic map.
Since the Grand Canyon region has undergone very little structural deformation since the deposition of these rocks, the original horizontal layering is still evident.